1. Field of the Invention
This invention relates generally to axial flow fans and, more particularly, to high efficiency axial flow fans that operate with reduced fan airflow turbulence and lower noise.
2. Description of the Related Art
Axial flow fans are used to ventilate and cool a great variety of areas, from personal computer cases to entire buildings. Axial flow fans include a multi-bladed impeller and may or may not include a shroud, which helps direct air past the blades. When the impeller is rotated by a fan motor, the pressure of the air passing through the blades is reduced, which causes continuous air movement toward the fan. The fan blades then raise the pressure and move the air out the rear of the fan. The moving air creates a steady flow that can be used to ventilate and cool particular areas.
Although axial flow fans are useful in ventilating and cooling, they conventionally are limited to a maximum 60% to 65% operating efficiency. For many devices that must be cooled, the component that requires the greatest amount of power for operation is the fan. Therefore, improving the efficiency of the fan can greatly improve the overall efficiency of such devices.
Much of the inefficiency of conventional fans is due to turbulence in the airflow as it is moved past the fan blades and as it exits the fan shroud. Turbulence is a random flow of air through the fan rather than a directed flow and, as a result, the fan motor must expend energy to overcome the turbulence and move the air through the fan. The turbulence also creates a great deal of unwanted noise. Fans used in small appliances and personal computers, for example, can produce noise that intrudes upon an otherwise relatively quiet environment. On a larger scale, care must be taken in large buildings to isolate the building occupants from the noise generated by the fans of the heating, ventilating, and air conditioning systems.
Another limitation of conventional fans is that most have a relatively narrow range of operating speeds and conditions. If the airflow through a fan is below the minimum required by the operating range of the fan, a stall condition can occur in which the air ceases to flow smoothly over the surfaces of the fan blades and violently separates from the blades. The violent separation of airflow greatly increases the turbulence, which reduces the airflow and pressure rise through the fan and increases the noise generated by the fan. In severe cases of stall, high vibration can be generated that can destroy the fan.
Finally, the accumulation of dirt, dust, insects, and the like on the fan blades reduces the performance of the fan by reducing both airflow and pressure rise through the fan. These reductions in turn decrease the fan efficiency and increase the fan noise level. Extreme cases of dirt, dust, and insect accumulation can reduce the airflow sufficiently to cause the fan to enter a severe stall that can destroy the fan if it is left to operate under such conditions.
From the foregoing discussion, it should be apparent that there is a need for an axial flow fan that provides increased efficiency and reduced noise over a relatively wide range of operating conditions. The present invention satisfies this need.